Metathesis reactions are used widely in chemical syntheses, e.g. in the form of ring-closing metatheses (RCM), cross metatheses (CM), ring-opening metatheses (ROM), ring-opening metathesis polymerizations (ROMP), cyclic diene metathesis polymerizations (ADMET), self-metathesis, reaction of alkenes with alkynes (enyne reactions), polymerization of alkynes and olefinization of carbonyls. Metathesis reactions are employed, for example, for the synthesis of olefins, for ring-opening polymerization of norbornene derivatives, for the depolymerisation of unsaturated polymers and for the synthesis of telechelic polymers.
A broad variety of metathesis catalysts is known, inter alia, from WO-A-96/04289 and WO-A-97/06185. They have the following general structure:
where M is osmium or ruthenium, the radicals R are identical or different organic radicals having a great structural variety, X1 and X2 are anionic ligands and the ligands L are uncharged electron-donors. In the literature, the term “anionic ligands” in the context of such metathesis catalysts always refers to ligands which, when they are viewed separately from the metal centre, are negatively charged for a closed electron shell.
In the last years metathesis reactions have become increasingly important for the degradation of nitrile rubbers also referred to as “NBR” for short, which is typically a copolymer or terpolymer of at least one α,β-unsaturated nitrile, at least one conjugated diene and, if appropriate, one or more further copolymerizable monomers.
Hydrogenated nitrile rubber, referred to as “HNBR” for short, is produced by hydrogenation of nitrile rubber. Accordingly, the C═C double bonds of the copolymerized diene units in HNBR are completely or partly hydrogenated. The degree of hydrogenation of the copolymerized diene units is usually in the range from 50 to 100%. HNBR is a specialty rubber which displays very good heat resistance, excellent resistance to ozone and chemicals and excellent oil resistance combined with very good mechanical properties, such as high abrasion resistance. For this reason, HNBR has found widespread use in a wide variety of applications and is used e.g. for seals, hoses, belts and damping elements in the automobile sector, also for stators, oil well seals and valve seals in the field of crude oil production and for numerous parts in the aircraft industry, the electronics industry, machine construction and shipbuilding.
Most HNBR grades which are commercially available on the market usually have a Mooney viscosity (ML 1+4 at 100° C.) in the range from 55 to 120, which corresponds to a number average molecular weight Mn (determination method: gel permeation chromatography (GPC)) against polystyrene standards) in the range from about 200,000 to 700,000. The polydispersity indices, “PDI”, measured (PDI=Mw/Mn, where Mw is the weight average molecular weight and Mn is the number average molecular weight, both determined by GPC against polystyrene standards), which give information about the width of the molecular weight distribution, are frequently 3 or above. The residual double bond content is usually in the range from 1 to 18% (determined by means of NMR or IR spectroscopy). However, it is customary in the art to refer to “fully hydrogenated grades” when the residual double bond content is not more than 0.9%.
The processability of HNBR grades with relatively high Mooney viscosities are subject to restrictions. For many applications HNBR grades which have a lower molecular weight and thus a lower Mooney viscosity are desirable since this significantly improves the processability.
Many attempts have been made in the past to shorten the chain length of HNBR by degradation. For example, a decrease in the molecular weight can be achieved by thermomechanical treatment (mastication), e.g. on a roll mill or in a screw apparatus (EP-A-0 419 952). However, functional groups such as hydroxyl, keto, carboxylic acid and carboxylic ester groups are introduced into the molecule by partial oxidation and, in addition, the microstructure of the polymer is altered substantially.
For a long time, it has not been possible to produce HNBR having a low molar mass corresponding to a Mooney viscosity (ML 1+4 at 100° C.) in the range below 55 or a number average molecular weight of about Mn<200,000 g/mol by means of established production processes since, firstly, a step increase in the Mooney viscosity occurs in the hydrogenation of NBR and secondly the molar mass of the NBR feedstock to be used for the hydrogenation cannot be reduced at will below a certain threshold since otherwise work-up in the industrial plants available is no longer possible because the rubber is too sticky. The lowest Mooney viscosity of an NBR feedstock which can be worked up without difficulties in an established industrial plant is about 30 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity of the HNBR obtained using such an NBR feedstock is in the order of 55 Mooney units (ML 1+4 at 100° C.). The Mooney viscosity is determined in accordance with ASTM standard D 1646.
In the more recent prior art, this problem is solved by reducing the molecular weight of the NBR before hydrogenation by degradation to a Mooney viscosity (ML 1+4 at 100° C.) of less than 30 Mooney units or a number average molecular weight of Mn<70,000 g/mol. The reduction in the molecular weight is achieved by metathesis in which low molecular weight 1-olefins are usually added. The metathesis of NBR is described, e.g. in WO-A-02/100905, WO-A-02/100941 and WO-A-03/002613. The metathesis reaction is advantageously carried out in the same solvent as the hydrogenation reaction so that the degraded nitrile rubber does not have to be isolated from the solvent after the degradation reaction is complete before it is subjected to the subsequent hydrogenation. The metathesis degradation reaction is catalyzed using metathesis catalysts which are tolerant to polar groups, in particular nitrile groups.
WO-A-02/100905 and WO-A-02/100941 describe a process comprising the degradation of NBR by olefin metathesis and subsequent hydrogenation to give HNBR having a low Mooney viscosity. Here, an NBR is reacted in the presence of a 1-olefin and specific complex catalysts based on Os, Ru, Mo, and W in a first step and hydrogenated in a second step. In this way, it is possible to obtain HNBR having a weight average molecular weight (Mw) in the range from 30,000 to 250,000, a Mooney viscosity (ML 1+4 at 100° C.) in the range from 3 to 50 and a polydispersity index PDI of less than 2.5. The metathesis of NBR is described to be carried e.g. using the catalyst bis(tricyclohexylphosphine)benzylideneruthenium dichloride (“Grubbs I”) or 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidenylidene)(tricyclohexyl-phosphine)(phenyl-methylene)ruthenium dichloride (“Grubbs II”) as shown below.

In terms of the molecular weight and the molecular weight distribution, the metathetic degradation using catalysts of the Grubbs (II) type proceeds more efficiently than when catalysts of the Grubbs (I) type are used.
In view of the many applications for metathesis catalysts their synthesis has gained increasing importance.
In J. Am. Chem. Soc. 2001, 123, 5372-5373 the synthesis of metal carbene complexes through the use of a sulfonium salt is reported which is in situ deprotonated to yield a sulfur ylide which then reacts with a metal precursor to yield the corresponding metal carbene complex. Specifically, Grubbs I catalyst is synthesized through this route. This route as shown in the following scheme does not involve ruthenium hydride complexes or vinyl sulfides and does not provide any teaching in this regard.

In J. Organomet. Chem. 2002, 641, 220 and Organometallics 2003, 22, 1986-1988 the synthesis of Grubbs-type complexes by reacting (PPh3)3RuHCl and a propargyl chloride is described. The resulting complex can then be transformed into Grubbs I catalysts by alkylidene and phosphine exchange.

In New. J. Chem. 2003, 27, 1451 the reaction of vinyl chloroformate with a ruthenium hydride starting material is described to yield a ruthenium ethylidene complex through CO2 elimination and chloride migration to the metal centre.

In Organometallics 1997, 16, 3867-3869 the reaction of a ruthenium hydride starting material with propargyl or vinyl halides is described to generate Grubbs-type complexes. It is further mentioned that the reactions of the ruthenium hydride starting material with alkenyl chlorides result in the formation of reactive alkyl carbenes but many by-products were observed which makes this procedure not synthetically viable.

In Angew. Chem. Int. Ed. 1998, 37, 2490, J. Am. Chem. Soc. 2003, 125, 2546, Angew. Chem. Int. Ed. 2009, 48, 5191-5194), Chem. Eur. J. 2010, 16, 3983-3993, and Organometallics 2010, 29, 3007-3011 several examples of olefin metathesis catalysts are shown including those depicted below bearing two N-heterocyclic carbenes as the neutral ligands. N-heterocyclic carbene ligands are in general often referred to as “NHC-ligands”. All of the papers either use Grubbs I or Grubbs II catalysts as the ruthenium starting material or use routes that have been described before to generate said Grubbs' catalysts. None of these NHCs feature side arms capable of binding to the metal to form a tridentate ligand.

In Organometallics 2003, 22, 3634-3636, J. Am. Chem. Soc. 2005, 127, 11882-11883, Organometallics 2006, 25, 1940-1944 and Organometallics 2009, 28, 944-946 the synthesis and metathesis activities of several ruthenium based catalysts including those depicted below is discussed where one or both the anionic ligands are substituted with either a monodentate or bidentate aryloxy group. These species are obtained using Grubbs-type complexes as the starting materials followed by anion exchange with the appropriate substrate.

In Organometallics 2004, 23, 280-287 the synthesis of ruthenium benzylidene complexes containing NHC ligands that have hydroxyalkyl chains is described. The neutral ligand can rearrange so that they are cis- rather than trans-disposed. In the presence of pyridine it was shown that the phosphine and one of the chlorides are displaced by 2 equivalents of pyridine and the hydroxyl group coordinates to the metal centre. The complexes below are synthesized using Grubbs I catalyst as the ruthenium starting material.

Organometallics 2013, 32, 29-46 describes the synthesis of ruthenium alkylidene complexes containing NHC ligands that have a pendant phosphine that binds to the metal upon complexation. A dichloro bridged dimer is first formed and, upon reaction with the appropriate substituted diazomethane, is then converted to the monomeric alkylidene ruthenium complex.

In Organometallics 2012, 31, 580-587 the synthesis and metathesis activity of tridentate bis-carbene ligands with a potential hemi-labile donor is described. The synthesis involves using first generation Grubbs catalyst as the ruthenium starting material.

In J. Am. Chem. Soc. 2013, 135, 3331-3334 the synthesis and Z-selective metathesis activity of a thiolate containing Grubbs-Hoveyda type catalyst as shown below is reported where the compound was synthesized starting with Grubbs-Hoveyda catalyst.

Summing up various catalysts are already available for metathesis reactions, however, many of them contain unfavourable ligands, are sometimes not sufficiently active and/or selective and, importantly, are difficult to prepare or may only be prepared with Grubbs I or II structures as starting materials.
Therefore, it was the object of the present invention to provide an active and thermally robust, novel catalyst which shows on the one hand catalytic activity for a broad variety of metathesis reactions and on the other hand should be accessible via a process route preferably not involving Grubbs I or II structures as starting materials.